Heat resistant polyester container and process for producing the same

ABSTRACT

A heat-resistant polyester container wherein the temperature T is not lower than 120° C. at a moment when the rate of contraction in the barrel portion of the polyester container represented by the following formula is 0.66%, 
 
Ratio of contraction (%)=(amount of contraction/gauge length)×100  (1) 
wherein the amount of contraction is measured from a test piece cut from the barrel portion of the polyester container so as to possess a gauge length of 20 mm in compliance with TMA without pre-loading while elevating the temperature at a rate of 3° C./min after 30° C. is exceeded. The polyester container exhibits excellent heat resistance, and enables the retort-sterilization to be executed after the food or beverage has been filled and sealed without permitting the barrel portion of the container to be deformed.

TECHNICAL FIELD

The present invention relates to a heat-resistant polyester containerobtained by biaxially draw-blow-molding a preform of a polyester resinsuch as a polyethylene terephthalate and to a method of producing thesame. More particularly, the invention relates to a polyester containerfor executing the retort-sterilization after the content has been filledwith a content and sealed.

Background Art

Polyester containers in the form of wide-mouthed bottles obtained byheating a preform of a polyester resin such as a polyethyleneterephthalate at a temperature of not lower than a glass transitionpoint (Tg) but not higher than a heat crystallization temperaturefollowed by the biaxial draw-blow molding, have been widely used forcontaining a variety kinds of foods, seasonings and beverages owing totheir excellent transparency, shock resistance and gas barrier property.

To impart the heat resistance to the polyester container, in general,the mouth portion of the preform of a polyester resin is suitably heatedso as to be crystallized, and is crystallized by the biaxial draw-blowmolding, and is, further, heat-set at a temperature of not lower thanthe crystallization temperature to remove distortion caused by thesecondary draw-blow molding. When placed under a temperature conditionof not lower than 70° C., however, the obtained polyester container isconspicuously deformed due to the contraction by heating.

To further impart the heat resistance to the polyester container, therehas been proposed a method according to which the mouth portion of thepreform of a polyester resin is suitably heated so as to becrystallized, the preform is biaxially draw-blow-molded by using aprimary blow mold to obtain a primary intermediate molded article whichis, then, heated to a sufficient degree in a shrink oven to obtain asecondary intermediate molded article, and the secondary intermediatemolded article is biaxially draw-blow-molded by using a secondary blowmold (see, for example, Japanese Examined Patent Publication (Kokoku)No. 7-67732).

According to this method, the primary intermediate molded articlebiaxially draw-blow-molded by the primary blow-molding is heated toforcibly form a secondary intermediate molded article by contraction,which is, then, blow-molded into the shape of a bottle without almostbeing draw-deformed.

In order to prevent expansion due to heat at the time of sterilizationand to prevent deformation due to a reduction in the pressure after thesterilization, however, the barrel portion of the heat-resistantpolyester container must form a variety of structures such as reducedpressure-absorbing panels (mirror portions) and reinforcing structuressuch as reinforcing beads and ribs. According to the method ofblow-molding a bottle without almost draw-deforming the secondaryintermediate molded article proposed in the above Japanese ExaminedPatent Publication (Kokoku) No. 7-67732, however, it is not possible toform the reduced pressure-absorbing panels or the reinforcing beads inthe barrel portion of the polyester container.

In particular, it is not possible to form the reduced pressure-absorbingpanels or the reinforcing beads in the polyester containers that must beretort-sterilized at high temperatures of not lower than 100° C. and,particularly, at 120° C. for 20 to 50 minutes after they have beenfilled with the contents, i.e., filled with foods such as infant's foodsand beverages such as coffee with milk.

According to the above method, further, the secondary intermediatemolded article and the final container have the same size or nearly thesame size. In biaxially draw-blow-molding the secondary intermediatemolded article by using the secondary metal mold, therefore, thereoccurs a so-called mold nipping; i.e., the surface of the secondaryintermediate molded article is nipped by the secondary blow mold.

The present applicant has previously proposed a polyester containerhaving an endothermic peak at the bottom portion of not lower than 150°C. but not higher than a melt starting point on a DSC curve by givingattention to the deformation and whitening of the bottom portion duringthe retort-sterilization at a high temperature and a method of producingthe same (Japanese Unexamined Patent Publication (Kokai) No.2001-150522). However, this method, too, still leaves a problem ofdeformation in the barrel portion during the retort-sterilization.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide a polyestercontainer having a high heat resistance, which features an excellentheat resistance, which enables the retort-sterilization to be effectedat a high temperature after it has been filled with food or beverage andsealed, and which does not permit the barrel portion of the container tobe deformed even after the retort-sterilization processing, and a methodof producing the same.

According to the present invention, there is provided a heat-resistantpolyester container wherein the temperature T is not lower than 120° C.at a moment when the coefficient of contraction in the barrel portion ofthe polyester container represented by the following formula is 0.66%,Coefficient of contraction (%)=(amount of contraction/gaugelength)×100  (1)

-   -   wherein the amount of contraction is measured from a test piece        cut from the barrel portion of the polyester container so as to        possess a gauge length of 20 mm by in compliance with TMA        without pre-loading while elevating the temperature at a rate of        3° C./min after 30° C. is exceeded.

It is desired that the polyester container has reducedpressure-absorbing panels in the barrel portion, and the coefficient ofcontraction and the temperature T are values at pole portions among thereduced pressure-absorbing panels.

According to the present invention, there is further provided a methodof producing a heat-resistant polyester container by biaxiallydraw-blow-molding a preform of a polyester resin by using a primarymetal mold to obtain a primary intermediate molded article, contractingthe primary intermediate molded article by heating to obtain a secondaryintermediate molded article, and biaxially draw-blow-molding andheat-setting the secondary intermediate molded article by using asecondary metal mold heated at 150 to 210° C., so that the thicknessreduction ratio of the barrel expressed by the following formula (2) isnot smaller than 5%,Thickness reduction ratio (%)={(t₁−t₂)/t₂}×100  (2)wherein t₁ is a thickness of the barrel portion of the secondaryintermediate molded article, and t₂ is a thickness of the barrel portionof the polyester container which is the molded article.

In the method of producing the polyester container of the presentinvention, it is desired that the polyester container that is obtainedhas reduced pressure-absorbing panels in the barrel portion, and thethickness reduction ratio is a value in the pole portions among thereduced pressure-absorbing panels formed in the barrel portion.

In the heat-resistant polyester container of the invention, thecoefficient of contraction and the temperature T are so defined that thetemperature is that of when the coefficient of contraction is 0.66%, thecoefficient of contraction being expressed by the above formula (1) fromthe results (FIG. 5) obtained by biaxially draw-blow-molding andheat-setting the beat contracted secondary intermediate molded articleby using a secondary metal mold to obtain a polyester container, andcutting the barrel portion of the polyester container into a test piecehaving a gauge length of 20 mm as shown in FIG. 4, and measuring thecoefficient of contraction of the test piece by the thermomechanicalanalysis (simply referred to as TMA) without pre-loading while elevatingthe temperature at a rate of 3° C./min after 30° C. is exceeded.

In particular, it is desired that the coefficient of contraction and thetemperature T at that moment are values in the pole portions among thereduced pressure-absorbing panels formed in the barrel portion. The poleportions among the reduced pressure-absorbing panels have a heatresistance inferior to that of the reduced pressure-absorbing panels.Therefore, by taking a measurement at these portions, the heat-resistantpolyester container of the present invention explicitly exhibits thesuperiority.

If the temperature T is not lower than 120° C. at a moment when thecoefficient of contraction is 0.66%, the volume coefficient ofcontraction of the polyester container can be lowered to be, forexample, not larger than 2%. That is, if the temperature T is not lowerthan 120° C. at a moment when the coefficient of contraction is 0.66%,it means that the barrel portion of the heat contracted secondaryintermediate molded article is biaxially drawn to a sufficient degreeand is heat-set by the secondary metal mold at the time of the secondaryblow-molding, exhibiting greatly improved heat resistance as compared tothe conventional polyester container. It is, therefore, made possible toeffect the retort-sterilization processing at a temperature of not lowerthan 100° C. and, particularly, not lower than 120° C. after thecontainer has been filled with a food such as infant's food or abeverage such as coffee with milk.

If the temperature T is lower than 120° C. at a moment when thecoefficient of contraction is 0.66%, on the other hand, the heatresistance becomes poor and can no longer sufficiently withstand theretort-sterilization at high temperatures as described above.

The coefficient of contraction of 0.66% is used as a criterion. This isbecause a maximum coefficient of contraction of about 0.66% is thecriterion whether the heat-resistant polyester container can be put to apractical use. If the coefficient of contraction is lower than the abovevalue even at temperatures higher than 120° C. which is aretort-sterilization temperature, it is obvious that excellent heatresistance is exhibited.

In the method of producing the heat-resistant polyester container of theinvention, further, if the thickness reduction ratio of the barrelportion becomes smaller than 5%, the temperature T becomes lower than120° C. at a moment when the coefficient of contraction is 0.66% and theobtained polyester container exhibits an inferior heat resistance givingrise to the occurrence of defective molding such as nipping by the moldduring the blow-molding and the occurrence of wrinkles. If the thicknessreduction ratio exceeds 30%, on the other hand, there occur suchproblems as rupture during the secondary blow-molding and deformationafter the container is taken out.

Concerning the thickness reduction ratio, it is desired that thethickness t₂ of the barrel portion of the polyester container is a valuein the pole portions among the reduced pressure-absorbing panels formedin the barrel portion. The pole portions among the reducedpressure-absorbing panels are thicker than the reducedpressure-absorbing panels. Therefore, by taking a measurement at theseportions, superiority is explicitly exhibited by the method of producinga heat-resistant polyester container of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view illustrating a heat-resistant polyester containerof the present invention;

FIG. 2 is a side view illustrating another heat-resistant polyestercontainer of the present invention;

FIG. 3 is a view illustrating a method of producing a heat-resistantpolyester container according to the present invention;

FIG. 4 is a view illustrating a test piece used for measuring thecoefficient of contraction in compliance with TMA; and

FIG. 5 is a diagram of a TMA curve illustrating the results of TMAmeasurement.

BEST MODE FOR CARRYING OUT THE INVENTION

[Polyester Container]

The heat-resistant polyester container of the present invention has afeature in that the temperature T is not lower than 120° C. when thecoefficient of contraction of the barrel portion represented by theabove formula (1) is 0.66%.

The heat-resistant polyester container of the present invention may bein the form of a wide-mouthed bottle-like container shown in FIG. 1 or abottle-like container shown in FIG. 2, though they are not to limit theinvention.

The wide-mouthed polyester container 1 shown in FIG. 1 comprises a widemouth portion 2, a shoulder portion 3, a bared portion 4 and a bottomportion 5. Reduced pressure-absorbing panels 6 are formed in the barrelportion. The polyester container has the mouth portion 2 crystallized bythe heat treatment, and has the shoulder portion 3, barrel portion 4 andbottom portion 5 heat-set by a secondary metal mold that will bedescribed later, and has a temperature T of not lower than 120° C. at amoment when the coefficient of contraction of the pole portions 7 amongthe reduced pressure-absorbing panels 6 in the barrel portion 4 is0.66%.

The polyester container 21 of the present invention shown in FIG. 2 isin the form of a bottle and comprises a mouth portion 22, a shoulderportion 23, an upper barrel portion 24 a, a lower barred portion 24 band a bottom portion 25. Reduced pressure-absorbing panels 26 and poleportions 27 are formed in the lower barrel portion 24 b, and areinforcing recessed bead 28 is formed in a boundary portion between theupper barrel portion 24 a and the lower barrel portion 24 b. In thecontainer illustrated in FIG. 2, too, the temperature T is not lowerthan 120° C. at a moment when the coefficient of contraction in the poleportions 27 among the reduced pressure-absorbing panels 26 is 0.66%.

As the material constituting the polyester container of the invention,there can be used any polyester resin provided it can be biaxiallydraw-blow molded and crystallized; i.e., there can be used thermoplasticpolyesters of the type of ethylene terephthalate, polyesters such aspolybutylene terephthalate and polyethylene naphthalate, or a blend ofpolyesters thereof and a polyolefin, polycarbonate or an arylate resin.In the ethylene terephthalate-type thermoplastic polyester used for thepolyester container of the present invention, the ethylene terephthalateunit occupies most of, generally, not less than 70 mol % of and,particularly, not less than 80 mol % of the ester recurring unit, and athermoplastic polyester resin is preferably used having a glasstransition point (Tg) of 50 to 90° C. and, particularly, 55 to 80° C.and a melting point (Tm) of 200 to 275° C. and, particularly, 220 to270° C.

As the thermoplastic polyester resin, there can be preferably used ahomopolyethylene terephthalate from the standpoint of heat resistance.However, there can be further used a copolymerized polyester containingan ester unit in small amounts other than the ethylene terephthalateunit.

As the dibasic acid other than the terephthalic acid, there can beexemplified aromatic dicarboxylic acids such as isophthalic acid,phthalic acid, and naphthalenedicarboxylic acid; alicyclic dicarboxylicacids such as cyclohexanedicarboxylic acid; and a combination of one ortwo or more of aliphatic dicarboxylic acids such as succinic acid,adipic acid, sebacic acid and docanedioic acid.

As the diol component other than the ethylene glycol, there can beexemplified propylene glycol, 1,4-butanediol, diethylene glycol,1,6-hexelyne glycol, cyclohexanedimethanol, and one or two or more kindsof ethylene oxide adducts of bisphenol A.

There can be further used a composite material obtained by blending theethylene terephthalate-type thermoplastic polyester with, for example, apolyethylene naphthalate, a polycarbonate or a polyarylate having arelatively high glass transition point in an amount of about 5 to about25% to thereby increase the strength of the material of when thetemperature is elevated. It is further allowable to use a polyethyleneterephthalate and the above-mentioned material having a relatively highglass transition point in a laminated form. As required, further, thepolyester resin may be blended with a lubricant, a reforming agent, apigment and an ultraviolet ray-absorbing agent.

The ethylene terephthalate-type thermoplastic polyester used in thepresent invention should at least have a molecular weight large enoughfor forming a film, and is of the injection grade or of the extrusiongrade depending upon the applications. Desirably, its inherent viscosityis in a range of 0.6 to 1.4 dl/g and, particularly, 0.63 to 1.3 dl/g.

The polyester container of the present invention can be constituted by asingle layer of the above-mentioned polyester resin, but may also beconstituted by a multiplicity of layers forming a gas barrier layer asan intermediate layer between the polyester resin layers which areforming the inner layer and the outer layer.

As the thermoplastic resin constituting the gas barrier layer, there canbe used, for example, an ethylene/vinyl alcohol copolymer, a polyamide,a polyvinylidene chloride resin, a polyvinyl alcohol or afluorine-contained resin.

As a particularly preferred gas barrier resin there can be exemplified asaponified product of an ethylene/vinyl acetate copolymer obtained bysaponifying an ethylene/vinyl acetate copolymer containing ethylene inan amount of 20 to 60 mol % and, particularly, 25 to 50 mol % such thatthe degree of saponification is not smaller than 96 mol % and,particularly, not smaller than 99 mol %.

Other preferred gas barrier resins may be polyamides having amide groupsin a number of 5 to 50 and, particularly, 6 to 20 per 100 carbon atoms,such as nylon 6, nylon 6,6, nylon 6/6,6 copolymer, metaxylene adipamide(MXD6), nylon 6,10, nylon 11, nylon 12, nylon 13, etc.

When the polyester container of the invention is constituted by a singlelayer of a polyester resin, the polyester resin may be blended with anoxidizing organic component and a transition metal catalyst such ascobalt to impart an oxygen-trapping function of the oxidizing organiccomponent by the oxidation of the transition metal catalyst. As theoxidizing organic component, there can be used a polyamide and,particularly, a xylylene group-containing polyamide.

In the multi-layer structure comprising the inner and outer layers ofthe polyester resin and the intermediate layer of the gas barrier layer,further, the resin constituting the gas barrier layer may be the onehaving oxygen-absorbing property to impart oxygen-absorbing property tothe gas barrier layer. The resin may be the one which utilizes theoxidation reaction of the resin, e.g., an oxidizing organic materialsuch as polybutadiene, polyisoprene, polypropylene or ethylene/carbonoxide copolymer, and the one obtained by mixing polyamides such as6-nylon, 12-nylon or metaxylylenediamine (MX) nylon with organic acidsalts containing a transition metal such as cobalt, rhodium or copper asan oxidizing catalyst, and a photosensitizer such as benzophene,acetophene or chloroketones. When the oxygen-absorbing material is used,the effect can be exhibited to a more enhanced degree upon theirradiation with a ray of high energy, such as ultraviolet rays orelectron rays.

Further, the gas barrier resin constituting the gas barrier layer maycontain an oxidizing organic component to produce oxygen-absorbingproperty without deteriorating the gas barrier property caused by thedeterioration of the gas barrier layer due to oxidation. As theoxidizing organic component, it is desired to use a polyene polymerderived from a polyene, and it is desired that a carboxylic acid, acarboxylic anhydride group or a hydroxyl group has been introducedthereto. As the functional group, there can be exemplified an acrylicacid, a methacrylic acid, a maleic acid, an unsaturated carboxylic acid,an anhydrous maleic acid or an anhydride of unsaturated carboxylic acid.As the transition metal catalyst, cobalt is preferred.

There can be further used, as a chief component, a combination of theabove-mentioned gas barrier resin constituting the gas barrier layerwith one or two or more of metal powders having reducing property, suchas a reducing iron powder, reducing zinc, a reducing tin powder, a metallow oxide and a reducing metal compound, which, as required, can also beused in combination with an assistant such as a hydroxide, a carbonate,a sulfite, an organic acid salt or a halide of an alkali metal or analkaline earth metal, or active carbon or active alumina. There can befurther used a high molecular compound having a polyhydric phenol in theskeleton, such as a polyhydric phenol-containing phenol/aldehyde resin.The oxygen-absorbing agent, usually, has an average particle size of notlarger than 10 μm and, particularly, not larger than 5 μm to maintaintransparency or semi-transparency.

The gas barrier resin layer, oxygen-absorbing resin and oxygen-absorbingmaterial may be blended with a filler, a coloring agent, a heatstabilizer, an aging stabilizer, an anti-oxidizing agent, an anti-agingagent, a photo stabilizer, an ultraviolet ray absorber, an antistaticagent, a lubricant such as a metal soap or a wax, and a reforming agent.

In employing the multi-layer constitution, further, an adhesive or anadhesive layer may be interposed among the resin layers.

[Method of Producing the Polyester Container]

According to the present invention, a method of producing aheat-resistant polyester container comprises biaxially draw-blow-moldinga preform of a polyester resin by using a primary metal mold to obtain aprimary intermediate molded article, contracting the primaryintermediate molded article by heating to obtain a secondaryintermediate molded article, and biaxially draw-blow-molding andheat-setting the secondary intermediate molded article by using asecondary metal mold heated at 150 to 210° C., so that the thicknessreduction ratio of the barrel portion expressed by the above-mentionedformula (2) is not smaller than 5%.

FIG. 3 is a view illustrating the method of producing a heat-resistantpolyester container of the present invention shown in FIG. 1. Namely, asillustrated in FIG. 3, the heat-resistant polyester container 1 of thepresent invention is obtained by crystallizing the mouth portion of apreform 10 of a polyester resin by a suitable heating means to impartheat resistance to the mouth portion, heating the preform at atemperature higher than a glass transition point (Tg), for example, at95 to 115° C., biaxially draw-blow molding the preform by a primarymetal mold to obtain a primary intermediate molded article 11 (step ofprimary blow), contracting the primary intermediate molded article 11 byheating to obtain a secondary intermediate molded article 12 from whichthe distortion formed by the biaxial draw-blow molding is removed (stepof contraction by heating), and biaxially draw-blow-molding andheat-setting the secondary intermediate molded article 12 by a secondarymetal mold heated at 150 to 210° C., so that the thickness reductionratio of the barrel portion expressed by the above formula (2) is notsmaller than 5% and, preferably, 5 to 30% (step of secondary blow/heatset). In the embodiment illustrated in FIG. 3, it is desired that thereduced pressure-absorbing panels 6 and the pole portions 7 are formedin the barrel portion, and the thickness reduction ratio expressed bythe above formula (2) is a value in the pole portions 7.

When the temperature is the same at the portions corresponding to thebarrel portion and to the bottom portion of the primary intermediatemolded article obtained by the molding, it is desired that thetemperature of the primary metal mold in the step of primary blow isfrom room temperature to 250° C. When the temperature of the metal moldexceeds 250° C., the material melts and the parting becomes defective.

It is desired that the temperatures of the primary metal mold aredifferent at portions corresponding to the barrel portion and to thebottom portion of the primary intermediate molded article obtained bythe molding, from the standpoint of stabilizing the contraction from theneck portion through up to the barrel portion. In this case, it isdesired that the temperature of the portion corresponding to the barrelportion is 70 to 250° C. When the temperature is lower than 70° C., theheating is not enough to stabilize the contraction to a sufficientdegree. When the temperature exceeds 250° C., on the other hand, thematerial melts and the parting becomes defective. It is desired that thetemperature of the portion corresponding to the bottom portion is fromroom temperature to 250° C. When the temperature exceeds 250° C., thematerial melts and the parting becomes defective.

It is desired that the drawing ratio in the step of primary blow is,generally, 1.5 to 5 times in the longitudinal direction, 1 to 4 times inthe transverse direction, and 3 to 20 times in terms of the area ratio.In the case of the wide-mouthed polyester container shown in FIG. 1, inparticular, it is desired that the drawing ratio is 2 to 4 times in thelongitudinal direction, 1 to 3.5 times in the transverse direction, and4 to 20 times in terms of the area ratio. In the case of the polyesterbottle as shown in FIG. 3, it is desired that the drawing ratio is 2.5to 4 times in the longitudinal direction, 1 to 4 times in the transversedirection, and 4 to 13 tomes in terms of the area ratio.

The heating conditions in the step of contraction by heating are suchthat the average temperature on the surface is controlled to be from 100to 250° C. When the average temperature is lower than 100° C., the shapecannot be imparted to a sufficient degree during the draw-blow moldingby using the secondary metal mold. When the average temperature exceeds250° C., the material melts and ruptures in the secondary blow, causingthe whitening due to heat crystallization.

It is desired that the temperatures of the secondary metal mold in thestep of secondary blow/heat set are 150 to 210° C. at portionscorresponding to the barrel portion and to the bottom portion of thesecondary intermediate molded article. When the temperature is lowerthan 150° C., the stress of molding is not relaxed to a sufficientdegree and the desired heat resistance is not obtained. When thetemperature exceeds 210° C., on the other hand, the parting becomesdefective, causing deformation and defective appearance when thepolyester container is taken out.

As required, further, a cooling blow is effected with the air of 20 to25° C. for 0.5 seconds to 3 seconds to prevent the deformation at thetime of taking the polyester container from the secondary metal mold.

It is desired that the drawing ratio in the step of secondary blow is,generally, 1 to 1.2 times in the longitudinal direction, 1.05 to 1.3times in the transverse direction, and 1.0 to 1.3 times in terms of thearea ratio. In the case of the wide-mouthed polyester container shown inFIG. 1, in particular, it is desired that the drawing ratio is 1 to 1.1times in the longitudinal direction, 1.05 to 1.2 times in the transversedirection, and 1.05 to 1.15 times in terms of the area ratio. In thecase of the bottle-like polyester container as shown in FIG. 3, it isdesired that the drawing ratio is 1 to 1.1 times in the longitudinaldirection, 1.05 to 1.25 times in the transverse direction, and 1.05 to1.2 times in terms of the area ratio.

It is further desired that the heat set is effected in the secondaryblow metal mold, generally, for 1 to 5 seconds.

In subjecting the preform of a polyester resin to the biaxial draw-blowmolding by using a primary metal mold, to the contraction by the heattreatment and to the biaxial draw-blow molding by using a secondarymetal mold, there have been proposed a variety of temperatures based onthe primary metal mold, secondary metal mold and contraction by the heattreatment. In order to obtain a highly heat-resistant polyestercontainer of which the barrel portion does not deform even after theretort-sterilization processing at a high temperature of not lower than100° C. and, particularly, at 120° C. for 20 to 50 minutes after havingbeen filled with food such as infant's food or beverage such as coffeewith milk, it is important in the present invention to adjust thedrawing ratios in the step of primary blow and in the step of secondaryblow and to adjust the time for contraction by heating in the shrinkoven, such that the thickness reduction ratio expressed by the aboveformula (2) is not smaller than 5% and, preferably, 5% to 30% at thetime of biaxially draw-blow-molding and heat-setting the thermallycontracted secondary intermediate molded article by using the secondarymetal mold.

In the method of producing the polyester container of the invention, apreform corresponding to the shape of a preform metal mold for injectionis produced by using a conventional injection-molding machine.

In the case of the multi-layer constitution, a multi-layer preformcorresponding to the shape of a preform metal mold for injection isproduced by using a co-injection molding machine to form the inner andouter layers of the polyester resin and to insert one or moreintermediate layers between the inner layer and the outer layer. Amulti-layer preform can be, further, produced by using a multi-stageinjection machine by, first, injection-molding a primary preform of apolyester resin by using a primary metal mold, transferring the primarypreform into a secondary metal mold in which a resin for constituting anintermediate layer is injected onto the surface thereof to obtain asecondary preform, and transferring the secondary preform to a tertiarymetal mold in which a polyester resin is injected onto the surfacethereof to form an outer layer.

A preform can be further produced by the compression-molding. In thiscase, a molten resin mass is fed into a female mold withoutsubstantially decreasing the temperature and is compression-molded by amale mold. In the case of forming a multiplicity of layers, a resin forforming the intermediate layer is provided in the molten resin mass thatconstitutes the inner and outer layers, and the molten resin mass is fedto the female mold without substantially decreasing the temperature andis compression molded by the male mold.

In order to impart heat resistance to the mouth-and-neck portion of thepreform obtained as described above, the mouth-and-neck portion iswhitened by crystallization through the heat treatment in the stage ofpreform.

The mouth-and-neck portion of the biaxially drawn portion may bewhitened by crystallization after the biaxial draw-blow molding.

EXAMPLES Example 1

A mouth portion of a preform made of a polyethylene terephthalate resinwas crystallized (whitened) by a suitable means and, then, the preformwas heated at 115° C. which was higher than the glass transition pointthereof. The preform was, then, biaxially draw-blow-molded into drawingratios of 2.8 times in the longitudinal direction, 2.8 times in thetransverse direction and 7.8 times in terms of an area by using theprimary metal mold heated at 160° C. at the portions corresponding tothe barrel portion and the bottom portion to obtain a primaryintermediate molded article of a circular shape in cross section havinga barrel diameter of 100 mm and a height of 100 mm larger than the finalpolyester container.

Next, the primary intermediate molded article was heated in an oven sothat the surface temperature was 180° C. on an average so as to bethermally contracted to thereby obtain a secondary intermediate moldedarticle of a circular shape in cross section having a thickness (t₁) inthe barrel portion of 0.5 mm (position 45 mm below the neck), a barreldiameter of 65 mm and a height of 90 mm.

Then, the secondary intermediate molded article was biaxiallydraw-blow-molded into 1.01 times in the longitudinal direction, 1.04times in the transverse direction and 1.05 times in terms of the area byusing a secondary metal mold heated at 150° C. at a portioncorresponding to at least the barrel portion 4, and was heat-set at theshoulder portion 3, barrel portion and bottom portion except the mouthportion 2 for 3 seconds, in order to obtain a wide-mouthedheat-resistant polyester container illustrated in FIG. 1 having athickness (t₂) in the pole portions 7 among the panels 6 of 0.45 mm(position 45 mm below the neck) (thickness reductionratio=(t₁−t₂)/t₂×=5%), a barrel diameter of 70 mm and a height of 95 mm.

In taking out the polyester container from the secondary metal mold,further, the cooling blow was effected to blow the air of 25° C. intothe container for one second.

Example 2

A polyester container was produced in the same manner as in Example 1with the exception of heating the secondary metal mold at a temperatureof 160° C. and selecting the drawing ratios in the biaxial draw-blowmolding to be 1.1 times in the longitudinal direction, 1.18 times in thetransverse direction and 1.3 times in terms of the area.

Example 3

A mouth portion of a preform made of a polyethylene terephthalate resinwas crystallized (whitened) by a suitable means and, then, the preformwas heated at 105° C. which was higher than the glass transition pointthereof. The preform was, then, biaxially draw-blow-molded into drawingratios of 2.8 times in the longitudinal direction, 3.5 times in thetransverse direction and 9.8 times in terms of an area by using theprimary metal mold heated at 130° C. at the portion corresponding to thebarrel portion and at 90° C. at the portion corresponding to the bottomportion to obtain a primary intermediate molded article of a circularshape in cross section having a barrel diameter of 85 mm and a height of210 mm larger than the final polyester container.

Next, the primary intermediate molded article was heated in an oven sothat the surface temperature was 180° C. on an average so as to bethermally contracted to thereby obtain a secondary intermediate moldedarticle of a circular shape in cross section having a thickness (t₁) inthe barrel portion of 0.48 mm (position 80 mm below the neck), a barreldiameter of 56 mm and a height of 158 mm.

Then, the secondary intermediate molded article was biaxiallydraw-blow-molded into 1.03 times in the longitudinal direction, 1.17times in the transverse direction and 1.2 times in terms of the area byusing a secondary metal mold heated at 180° C. at portions correspondingto at least the barrel portions 24 a, 24 b, and was heat-set at theshoulder portion 23, barrel portion 24 and bottom portion 25 except themouth portion 22 for 2 seconds, in order to obtain a bottle-likeheat-resistant polyester container illustrated in FIG. 3 having athickness (t₂) in the pole portions 37 among the panels 26 of 0.38 mm(position 80 mm below the neck) (thickness reductionratio=(t₁−t₂)/t₂×100=20%), a barrel diameter of 70 mm and a height of165 mm.

In taking out the polyester container from the secondary metal mold,further, the cooling blow was effected to blow the air of 25° C. intothe container for 0.8 seconds.

Example 4

A polyester container was produced in the same manner as in Example 3with the exception of heating the secondary metal mold at a temperatureof 210° C. and selecting the drawing ratios in the biaxial draw-blowmolding to be 1.1 times in the longitudinal direction, 1.09 times in thetransverse direction and 1.1 times in terms of the area.

Comparative Example 1

A polyester container was produced in the same manner as in Example 1with the exception of heating the secondary metal mold at a temperatureof 130° C. and selecting the drawing ratios in the biaxial draw-blowmolding to be 1 time in the longitudinal direction, 1.02 times in thetransverse direction and 1.02 times in terms of the area.

Comparative Example 2

A polyester container was produced in the same manner as in Example 3with the exception of heating the secondary metal mold at a temperatureof 140° C. and selecting the drawing ratios in the biaxial draw-blowmolding to be 1.01 times in the longitudinal direction, 1.02 times inthe transverse direction and 1.03 times in terms of the area.

(Evaluation)

[Coefficient of Contraction]

Test pieces having a gauge length of 20 mm and a width of 3 mm as shownin FIG. 4 were cut out from the pole portions among the reducedpressure-absorbing panels in the barrel portions of the polyestercontainers, and were put to the TMA measurement.

By using a dynamic viscosity measuring apparatus (DMS-6100, SeikoInstruments Inc.), the test pieces were put to the TMA measurement undera pre-loading of 0 (N) on the test pieces and under a condition ofelevating the temperature at a rate of 3° C./min.

In FIG. 5, the X-axis represents the temperature (° C.) of the testpiece, and the Y-axis represents the coefficient of contraction of thetest piece. The coefficient of contraction at 30° C. was set to be 0%,and the temperature T at a moment when the coefficient of contractionwas 0.66% was confirmed from the amount of contraction/gauge length.

The results were as shown in Table 1.

[Heat Resistance]

Polyester containers were filled with coffee with milk maintained at 25°C., sealed with plastic screw caps made of a polypropylene, and wereretort sterilized at 120° C. for 30 minutes. Table 1 illustrates thecoefficients of contraction of the containers. TABLE 1 EvaluationTemperature Heat T of when resistance Drawing ratio the (volumeTemperature of secondary ratio of ratio of Moldability (reduced ofsecondary metal mold Thickness contraction con- pressure-absorbing metalmold (longi × reduction becomes traction panels, pole (° C.) trans. =area) ratio (%) 0.66% %) portions, etc.) Ex. 1 150 1.01 × 1.04 = 1.05 5128.1 1.8 good Ex. 2 160 1.1 × 1.18 = 1.3 30 123.7 2 good Ex. 3 180 1.03× 1.17 = 1.2  20 150.3 1.5 good Ex. 4 210 1.01 × 1.09 = 1.1  10 201.80.8 good Comp. 130   1 × 1.02 = 1.02 2 102.3 9.6 reduced pressure- Ex. 1absorbing panels were poorly molded and the article was nipped by themold Comp. 140 1.01 × 1.02 = 1.03 3 116.0 3.9 reduced pressure- Ex. 2absorbing panels were poorly molded arid the article was nipped by themold

Industrial Applicability

The heat-resistant polyester container of the present invention exhibitsexcellent heat resistance and can be put to the retort-sterilizationprocessing at high temperatures after having been filled with foods orbeverages and sealed. In particular, the heat-resistant polyestercontainer of the invention is suited for retort-sterilizing such foodsas infant's foods or beverages such as coffee with milk at hightemperatures of not lower than 100° C. and, particularly, at 120° C. for20 to 50 minutes.

According to the production method of the present invention, further, itis allowed to easily produce a polyester container having a greatlyimproved heat resistance as compared to the conventional polyestercontainers.

1. A heat-resistant polyester container wherein the temperature T is notlower than 120° C. at a moment when the coefficient of contraction inthe barrel portion of the polyester container represented by thefollowing formula is 0.66%,Ratio of contraction (%)=(amount of contraction/gauge length)×100  (1)wherein the amount of contraction is measured from a test piece cut fromthe barrel portion of the polyester container so as to possess a gaugelength of 20 mm in compliance with TMA without pre-loading whileelevating the temperature at a rate of 3° C./min after 30° C. isexceeded.
 2. A heat-resistant polyester container according to claim 1,wherein the polyester container has reduced pressure-absorbing panels inthe barrel portion, and the coefficient of contraction and thetemperature T are values at pole portions among the reducedpressure-absorbing panels.
 3. A method of producing a heat-resistantpolyester container by biaxially draw-blow-molding a preform of apolyester resin by using a primary metal mold to obtain a primaryintermediate molded article, contracting the primary intermediate moldedarticle by heating to obtain a secondary intermediate molded article,and biaxially draw-blow-molding and heat-setting the secondaryintermediate molded article by using a secondary metal mold heated at150 to 210° C., so that the thickness reduction ratio of the barrelportion expressed by the following formula (2) is not smaller than 5%,Thickness reduction ratio (%)={(t₁−t₂)/t₂}×100  (2) wherein t₁ is athickness of the barrel portion of the secondary intermediate moldedarticle, and t₂ is a thickness of the barrel portion of the polyestercontainer which is the molded article.
 4. A method of producing aheat-resistant polyester container according to claim 3, wherein thepolyester container has reduced pressure-absorbing panels in the barrelportion, and the thickness reduction ratio is a value in the poleportions among the reduced pressure-absorbing panels formed in thebarrel portion.